Dynamic speaker management (“DSM”) uses the principle of dynamically maximizing speaker cone excursion at the expense of modifying the frequency response of the replayed material, based upon the content of the content of the program material. DSM allows dynamic boosting of portions of the frequency spectrum to maximize sound pressure level or “SPL.” Ultimately, this is constrained by cone excursion and/or amplifier drive capability. An example functionality of a DSM system is illustrated in FIG. 1.
Gain reduction or boost in DSM systems is typically implemented using multi-band limiters, where the spectrum is split into defined segments, each of which is maximized in amplitude and then are recombined to constitute the output signal. FIG. 2 is a block diagram illustrating a simplified signal processing path of a typical implementation. The incoming signal is split into various frequency bands, 1 through n. Each band is then passed through its own limiter function to maximize signal content within set limitations. The DSM system then applies variable thresholds to the limiters by extracting speaker parameters in real time through current and voltage sensing of the output signal. Each limiter makes independent decisions where the gain is limited, in each, to a known level.
The dynamic, program dependent alteration of frequency works well with single (“mono”) speaker implementations. However in stereo implementations, the audio stereo image can be adversely affected if DSM is applied independently to each channel. For each channel, the instantaneous frequency responses could be radically different depending on content, with the effect being worsened for multichannel implementations. This effect is referred to as “image shift” or “image distortion.”
The image shift problem has been addressed in the prior art with a technique known as “sidechain linking”, where corresponding limiters of each stereo channel are permanently linked together so that the gain reduction for each frequency band in each channel is always the same. This cures image shift, but at the downside of a significant decrease in overall SPL. That is, if content in one channel does not need to be attenuated, but is being overridden by content in its linked channel, then the SPL maximization is not being utilized fully for the channel not needing attenuation.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.